Metallic surface coatings are commonly applied to electronic devices and decorative objects to provide corrosion protection and other desired functional properties. Electronic devices comprising copper or copper alloys typically comprise metallic surface coatings which provide corrosion protection, high surface contact resistance, and wear resistance.
In the electronics industry, metallic connectors and lead frames may be coated with surface coatings comprising one or more metallic layers. For example, a metallic surface coating may comprise a base metal underlayer and a precious metal overlayer. In connector manufacture, the base metal underlayer, such as a nickel underlayer, is coated over the copper or copper alloy substrate. The base metal serves as a diffusion barrier. The precious metal overlayer, such as gold, palladium, silver, or alloys thereof, is then coated over the base metal underlayer coating. The precious metal overlayer provides corrosion resistance, wear resistance, and high conductivity. In one metallic surface coating, a nickel underlayer increases the hardness of a gold overlayer. This metallic surface is commonly referred to as “nickel-hardened gold” or simply, “hard gold.” In lead frame manufacture, the nickel substrate may be first coated with a palladium layer, which is then over-coated with a gold top coat. Variations on these coatings involve base metal alloy underlayers, precious metal alloy overlayers, and metallic surface coatings comprising two or more base metal underlayers and/or two or more precious metal overlayers.
An obvious disadvantage to the use of precious metals such as gold and palladium is cost. A cost effective connector uses a precious metal coating layer which is as thin as possible, without sacrificing the desired functional properties. Accordingly, the industry typically employs precious metal layer on the order of about 1.0 μm thick on electronic connectors. Thinner layers suffer from the disadvantage of highly increased porosity in the coating. Over time in service, the thin layers having a high degree of porosity are ineffective against base metal and copper diffusion to the surface. In a corrosive environment, the exposed base metal and copper will corrode and the corrosion product(s) can migrate onto the coating surface and deteriorate the surface contact conductivity. Moreover, a thin precious metal layer can wear off during application and shorten the connector's useful lifetime.
For many years in the manufacture of printed circuit boards (PCB), bare boards comprising copper circuitry were finished with eutectic tin-lead solder coating according to the Hot Air Solder Leveling (HASL) process. Due to the Restriction of Hazardous Substances (RoHS) directive, the industry has moved away from using lead as a component of the final finish of bare boards. One alternative final is electroless nickel-immersion gold (ENIG). Another alternative final finish is an immersion displacement layer of silver directly over copper circuitry.
With regard to ENIG alternative final finishes in PCB manufacture, gold may be applied as a metallic surface coating over copper substrates for corrosion resistance and increased wear resistance. Typically, gold is not deposited directly on the copper substrate, but rather on an intervening base metal underlayer. The base metal underlayer, typically electrolessly deposited nickel, is deposited on the copper or copper alloy substrate. The base metal serves as a diffusion barrier. The precious metal overlayer, such as gold, palladium, or alloys thereof, or layers thereof is then deposited, typically by an immersion displacement method, over the base metal underlayer coating. The precious metal overlayer (i.e., gold, palladium, a gold-palladium alloy, or a layer of palladium topped with a layer of gold) provides corrosion resistance, wear resistance, and high conductivity. In the conventional electroless nickel-immersion gold method, an electrolessly deposited nickel underlayer increases the hardness of the immersion plated gold overlayer. ENIG is vulnerable to common pollutants and is sensitive to high humidity and tends to fail due to corrosion.
Another alternative final finish is an immersion displacement layer of silver directly over copper circuitry. The silver is generally deposited by immersion displacement plating, in which silver ions present in the plating composition come into contact with and are reduced by surface copper atoms, according to the following reaction:Cu(s)+2Ag+(aq)→Cu2+(aq)+2Ag(s).The reduction-oxidation reaction reduces silver ions to silver metal and forms an adhesive silver layer over the copper substrate. The process is self-limiting in that once the copper surface is covered with a layer of silver, copper atoms are no longer accessible to reduce additional silver ions. Typical thicknesses of silver immersion displacement films over copper may range from about 0.05 and about 0.8 microns. See, for example, U.S. Pat. Nos. 5,955,141; 6,319,543; 6,395,329; and 6,860,925, the disclosures of which are hereby incorporated by reference as if set forth in their entireties.
A particular problem observed with immersion-plated precious metal coatings, e.g., silver and gold, as alternative final finishes in PCB manufacture is creep corrosion of copper salts at certain bare copper interfaces between copper and precious metal. For example, immersion silver displacement plating processes may not sufficiently coat copper wiring in PCB, particularly at plated through holes and high aspect ratio blind vias. Corrosion at these locations manifests itself as an annular ring surrounding the vias and plated through holes.
Moreover, silver is susceptible to sulfidation by reduced sulfur compounds (e.g., hydrogen sulfide) present in the environment, particularly at paper processing plants, rubber processing plants, and high pollution environments. Sufficient sulfidation of silver can result in localized pores, which may expose copper to the environment. Humidity and environmental pollutants can oxidize and sulfidize the copper, forming copper salts that may creep through pores in the silver layer.
A need continues to exist in the connectors industry for a metallic coating surface which uses as little precious metal as possible while still retaining advantages the precious metal overlayer provides. Moreover, a need continues to exist for ENIG and immersion silver final finishes over copper circuitry in PCB manufacture that are less vulnerable to corrosion.